1. Field of the Invention
The present invention relates to a braking force distribution apparatus (front-rear braking force distribution apparatus) and a braking force distribution method which control the ratio of braking force applied to rear wheels to that applied to front wheels when the vehicle is braked.
2. Description of the Related Art
In an ordinary vehicle, when a driver steps on the brake pedal, braking forces are applied to front and rear wheels. In such a case, the ratio of the braking force applied to the rear wheels (hereinafter referred to as “rear wheel braking force”) to the braking force applied to the front wheels (hereinafter referred to as “front wheel braking force”) is previously set for each vehicle model. When braking is performed in this manner, the load of the vehicle moves toward the front of the vehicle, and thus, the rear wheels become more likely to lock than the front wheels. That is, rear-wheel preceding lock occurs.
A conventionally known braking force distribution apparatus is an apparatus for preventing such rear-wheel preceding lock. More specifically, on the basis of the understanding that the angular speed of front wheels detected by means of a wheel speed sensor for the front wheels is in proportion to the rotational speed of the front wheels (the peripheral speed of tires of the front wheels; hereinafter referred to as “front-wheel wheel speed”), the conventional braking force distribution apparatus obtains the front-wheel wheel speed from the detected front-wheel angular speed. Similarly, on the basis of the understanding that the angular speed of rear wheels detected by means of a wheel speed sensor for the rear wheels is in proportion to the rotational speed of the rear wheels (the peripheral speed of tires of the rear wheels; hereinafter referred to as “rear-wheel wheel speed”), the conventional braking force distribution apparatus obtains the rear-wheel wheel speed from the detected rear-wheel angular speed.
Then, when the apparatus determines that the rear-wheel wheel speed is lower than the front-wheel wheel speed by a predetermined amount, the apparatus increases more gradually, holds, or gradually reduces the brake hydraulic pressure supplied to a rear-wheel braking apparatus (wheel cylinders for the rear wheels), as compared with the case of ordinary braking. As a result, the ratio of the rear wheel braking force to the front wheel braking force decreases, and the decreasing rate of the rear-wheel wheel speed gradually decreases and becomes smaller than that of the front-wheel wheel speed, whereby the rear-wheel wheel speed approaches the front-wheel wheel speed. Subsequently, when the apparatus determines that the difference between the rear-wheel wheel speed and the front-wheel angular speed has become sufficiently small, the apparatus increases the break hydraulic pressure applied to the rear-wheel braking apparatus. As a result, the ratio of the rear wheel braking force to the front wheel braking force returns to the predetermined value.
In other words, the conventional braking force distribution apparatus is designed to operate during braking of the vehicle so as to control the rear wheel braking force in such a manner that the ratio of the rear wheel braking force to the front wheel braking force decreases in accordance with the degree of slippage of the rear wheels as compared with the front wheels. Notably, such braking force distribution control is also called EBD (Electronic Brake force Distribution) control (see, for example, Japanese Patent Application Laid-Open (kokai) No. H5-278585 (paragraph No. 0005, FIG. 1).
Incidentally, even when a wheel rotates at a constant angular speed, the wheel speed changes depending on the dynamic load radius of the tire. Further, the dynamic load radius changes greatly depending on the internal pressure (air pressure) of the tire.
However, the above-described conventional braking force distribution apparatus obtains the front-wheel wheel speed and the rear-wheel wheel speed without consideration of the internal pressures of the respective tires, and on the basis of their relation determines whether to perform braking force distribution control. Therefore, when the internal pressures of the respective tires change, a large error is produced between the obtained front-wheel and rear-wheel wheel speeds and the actual front-wheel and rear-wheel wheel speeds, whereby the braking force distribution control may be started earlier than the timing at which the braking force distribution control ought to be performed, or may fail to be started when such timing has come.